1. Field of the Invention
Glass sheets are tempered by a process involving heating glass sheets above the strain point and approaching the softening point of the glass and then substantially cooling the heated glass sheets. Typical apparatus for tempering glass sheets comprises a tunnel-type heating furnace followed by a cooling station disposed in end-to-end relation with said furnace.
When glass sheets are tempered or heat strengthened, it is necessary to heat the glass sheets to a temperature above that at which the contour is changed by a deformation stress on contact with solid members. Where it is desired to strengthen glass sheets, it is further necessary to rapidly cool the glass sheets from such a deformation temperature to a lower temperature below the annealing range of the glass. The effectiveness of such strengthening is improved by an improvement in the control of the temperature to which the glass sheets are heated before they are rapidly chilled. This includes controlling the relative temperature of the top and bottom glass surfaces and the temperature profile across the dimension of the sheet transverse to its path of movement through the heating furnace and cooling station.
Glass is known to be strong in compression and weak in tension. Tempering glass increases the compression stress at the surface, thus increasing the resistance of tempered glass to fracture on impact compared to untempered glass. Furthermore, in the less frequent times that tempered glass fractures on impact, it forms smaller fragments that are less harmful than the relatively large fragments with jagged edges that result when untempered glass is broken.
There are three well-known systems for handling glass sheets during tempering. In one of these systems, glass sheets are suspended by one or more sets of tongs in a vertical disposition during their conveyance through a heating furnace and a cooling station. The tongs tend to penetrate the heat-softened glass and impart poor optical properties in the vicinity of the tong support areas.
Glass sheets are also handled by conveyance on spaced, rotating rollers disposed with their peripheral surfaces forming a horizontal path for transporting glass sheets through an elongated heating furnace and a cooling station. The need to space the rollers from one another to permit radiant energy to impinge on the lower glass surface causes the glass to sag and develop an optical defect. This defect has resulted in a desire to develop a technique for handling glass sheets in which their engagement by solid members which tend to distort the glass with its heat-softened properties is at a minimum. Such a technique is the so-called gas hearth method in which glass sheets, at least during the portion of the heating and cooling cycle when they are sufficiently hot to distort readily when their major surface contacts a solid member, are supported on a gaseous bed and engaged along a side edge only so as to minimize the portion of the glass sheet surface that engages a solid member during the critical portion of its thermal treatment when the glass is most liable to distortion.
When glass sheets are tempered on the gas hearth, if their upper and lower surfaces are heated to different temperatures, the glass tends to warp. If the glass warps into a configuration that is concave in elevation in a direction transverse to the path of glass movement, the lower central portion of the glass comes into engagement with the upper surface of the apertured bed which supplies hot gas for supporting the glass sheet during its thermal treatment, thereby developing surface scratches that develop into optical defects. If the concave warp is sufficiently severe, the glass will stick on the hearth bed and stop the flow of glass sheets through the furnace.
In a typical gas hearth operation, the glass is supported in a transversely oblique plane oriented approximately 5.degree. from the horizontal in a direction transverse to the direction of glass sheet movement and rotating discs are provided to engage the lower edge of the glass to propel the glass sheets by friction in a forward direction through the critical portion of the conveyor. The temperature profile of the glass sheets transverse to their path of movement along the gas hearth is very important. The temperature must be substantially uniform, although the edge portion engaged by the rotating discs that propel the glass sheets forward through the gas hearth heating furnace may be at a slightly lower temperature such that the engaged edge is not unduly distorted by contact with the rotating driving discs.